Various approaches of dual port modulation of a frequency synthesizer loop are utilized to obtain a flat frequency response. Dual port modulation involves appropriately adding modulating signals to the phase-locked loop (PLL) to achieve balanced high-pass and low-pass responses. Together, these responses result in uniform modulation below, through and above the loop unity gain frequency of the phase-locked loop.
The most fundamental method of dual port modulation requires simultaneous modulation of the reference oscillator and the voltage controlled oscillator (VCO). One arrangement feeds the modulating signal in both at the VCO and ahead of the phase comparator. An implementation more readily integratable has a phase modulation circuit in the reference path of the phase comparator and a modulated VCO.
However, this method may be inappropriate or difficult to achieve in some applications. For example, the reference oscillator can not be modulated if a high stability crystal oscillator is used. If the goal is to add modulation to a currently available PLL, that integrated circuit may not have the phase modulator built-in already or the required terminals accessible for modulation. Since modulation is in the reference path of the phase comparator, the phase modulator can not be completely disabled if modulation is no longer wanted.
Another method applies modulation to both the input of a voltage controlled oscillator (VCO) and, through a summing voltage network, to the output of a phase detector. Although this method can achieve wide deviation capability for a modulating signal, "balancing" (each modulation port must provide the same magnitude of deviation sensitivity and in the same direction in order to maintain the flat overall response) between the two ports is hard to achieve due to variations in components' sensitivities to temperature and time delays (thereby increasing cost).
Accordingly, there exists a need for a modulation arrangement for a phase-locked loop frequency synthesizer which is capable of wide deviation while minimizing cost and components by having modulation added-on.